Warehouse: Implement/Integrate statistics

[dstufft]

The new design has a pane for statistics, however we need to actually implement and integrate them.

Relevant statistics to show:

• Download counts

[nlhkabu]

This tab will be removed for launch (see #1310) - we should consider this a long term 'todo' :)

For reference, this is the design.

[brainwane]

Related: #3400 & #3434 on https://libraries.io/pypi/ and https://packaging.python.org/guides/analyzing-pypi-package-downloads/ . The right thing to do here, like in #991, might be to aid @andrew in getting download stats into libraries.io (https://github.com/librariesio/libraries.io/issues/1916), then pull a selected stat or two into the display in Warehouse but still point people to libraries.io and the Google BigQuery dataset for more details.

cc @a-wakeel and @micurd who opened related issues.

[andrew]

Related issue on the Libraries.io repo about download stats, will be put back on the roadmap within a few months I expect: https://github.com/librariesio/libraries.io/issues/53

[di]

https://github.com/pypa/pypi-legacy/issues/254 says:

as a package owner/maintainer, it'd be fantastic if there was any way to get something more fine-grained than "downloads per month/week/day".

i recognize that privacy concerns that put some limitations on here, but a few things i think might still be doable, in priority order:

1/ what package install led to me being installed?

this one would be huge -- if i'm maintaining a package that's a common dependency for several packages, which ones are driving my installs? in particular, if i need to change/deprecate something, which dependencies am i most likely to break?

2/ breakdowns by OS/python version

the biggest win here would of course be py2 vs. py3 -- how many of my users have already switched? on top of that, OS information (even just linux/mac/win) would be good, esp. around testing priorities.

3/ support for user-provided data in the install request

this one's quite general, but opens up all kinds of uses. my motivation: i'd love to know (eg) all installs that come as part of a google-owned tool. these are different from a "how many people use my package" POV, and in this case, any nontrivial number probably points to somewhere we should be caching. but i can't see the breakdown, so i don't know.

[crflynn]

@di I currently support some of these breakdowns at pypistats.org; I can contribute here by migrating some functionality to warehouse with some guidance. Is there a detailed feature spec for displaying these metrics in warehouse?